Docker compat: Return null when swappiness is -1

[inknos]

Fixes: #23824

Does this PR introduce a user-facing change?

`podman inspect` returns MemorySwappiness: null when it's not set

[openshift-ci]

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[inknos]

I speak a very broken Go, and I am not sure whether this is the right place to fix it, so I need some expertise here. @baude @Luap99

(It makes some sense in my head though)

[inknos]

@Luap99 makes more sense now.

However, I am checking it on my system and MemorySwappiness is 0...

$ ./bin/podman inspect practical_germain | grep Swap
               "MemorySwap": 0,
               "MemorySwappiness": 0,

Not sure if I am missing something or if this is part of a bigger problem. Shouldn't it be -1 on systems with cgroupsv2?

$ grep cgroup /proc/filesystems
nodev   cgroup
nodev   cgroup2

[Luap99]

@Luap99 makes more sense now.

However, I am checking it on my system and MemorySwappiness is 0...

$ ./bin/podman inspect practical_germain | grep Swap
               "MemorySwap": 0,
               "MemorySwappiness": 0,

Not sure if I am missing something or if this is part of a bigger problem. Shouldn't it be -1 on systems with cgroupsv2?

$ grep cgroup /proc/filesystems
nodev   cgroup
nodev   cgroup2

Good question, I guess so.

@giuseppe @mheon WDYT?

[giuseppe]

looks like a bug. We set the memory swappiness only when there is a memory limit:

$ podman run --rm -d --name foo alpine sleep 10
3f97720574b8eab140107b2f89e7cf7e6f3944af9c780369d47f51f9e22fde2b
$ podman inspect foo | grep Swap
               "MemorySwap": 0,
               "MemorySwappiness": 0,

$ podman run --rm -d --name foo --memory 1G alpine sleep 10
6c50d6921e80e7278e191128db74e01e2f38793a73fe7d3c25fcf4fba1c1a572
$ podman inspect foo | grep Swap
               "MemorySwap": 2147483648,
               "MemorySwappiness": -1,

[inknos]

looks like a bug. We set the memory swappiness only when there is a memory limit:

thanks. I can look for a fix for it

[giuseppe]

I think we could just error out when users try to set it on cgroup v2 (crun does it anyway), and hard code it to -1 for inspect.

@Luap99 @mheon what do you think?

[inknos]

$ podman run --rm -d --name foo --memory-swappiness 4 alpine sleep 3
Error: OCI runtime error: crun: cannot set memory swappiness with cgroupv2

It errors out already no?

[mheon]

Might be cleanest to hard-code to -1 just for inspect, the resource limit code in SpecGen is pretty ugly

[github-actions]

A friendly reminder that this PR had no activity for 30 days.

[baude]

This PR has been marked for 5.3 inclusion but it must be merged prior to Nov 5 for inclusion in 5.3 RC3. PRs not merged by that date are considered on a case by case basis for backporting.